function creerRequete() {
  var requete = null;	
  try {
    requete = new XMLHttpRequest();
  } catch (essaimicrosoft) {
    try {
      requete = new ActiveXObject("Msxml2.XMLHTTP");
    } catch (autremicrosoft) {
      try {
        requete = new ActiveXObject("Microsoft.XMLHTTP");
      } catch (echec) {
        requete = null;
      }
    }
  }

  if (requete == null) {
    alert("Impossible de créer l'objet requête!");
  } else {
    return requete;
  }
}

var requete = creerRequete();